A conventional image coding apparatus for coding a video sequence divides each picture included in the video sequence into a plurality of blocks. Then, the conventional image coding apparatus performs coding for each of the blocks in the raster scan order. As a result, the conventional image coding apparatus generates a coded stream (a bit stream) by coding and compressing the video sequence. Then, a conventional image decoding apparatus decodes this coded stream on a block-by-block basis in the raster scan order to reproduce the pictures of the original video sequence.
The conventional image coding methods include the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) H.264 standard (see Non Patent Literature 1, for example). When the images coded according to the H.264 standard are to be decoded, the conventional image decoding apparatus firstly reads the coded stream. After decoding each piece of header information, the conventional image decoding apparatus performs variable-length decoding. Then, the conventional image decoding apparatus performs inverse quantization and inverse frequency transform on coefficient information obtained by the variable-length decoding. As a result, a difference image is generated.
Next, according to a block type obtained by the variable-length decoding, the conventional image decoding apparatus performs intra-picture prediction (which may also be referred to as intra prediction) or inter-picture prediction (which may also be referred to as inter prediction). As a result, the conventional image decoding apparatus generates a prediction image. After this, the conventional image decoding apparatus performs an image reconstruction process by adding the difference image to the prediction image. Then, the conventional image decoding apparatus decodes the current image to be decoded (i.e., the coded image) by performing deblocking filtering on the reconstructed image obtained by the image reconstruction process.
According to the H.264 standard, the size of each block is always 16 by 16 pixels. In general, each of the decoding processes is performed for each 16-by-16-pixel block as well.
In recent years, super high resolution displays of, for example, 4K2K (3840 pixels by 2160 pixels) have been developed. For this reason, the number of pixels included in images to be processed is expected to be increasingly higher. Thus, the image coding apparatus that performs coding and decoding always on a 16-by-16-pixel block basis according to the H.264 standard needs to code a larger number of blocks and, as a result, the coding efficiency is reduced.
With this being the situation, techniques proposed as next-generation image coding standards include a technique that solves the stated problem (see Non Patent Literature 2, for example). With this technique, the size of a coding unit block according to the conventional H.264 standard is made variable. Thus, the image coding apparatus employing this technique can code an image for each block that is larger than the conventional 16-by-16-pixel unit block. Therefore, the coding efficiency increases.
Moreover, a pipeline processing technique is also available that increases the decoding efficiency by performing, in parallel, the processes for decoding the coded stream (see Patent Literature 1, for example). It should be noted that an apparatus which processes a coded stream, such as an image coding apparatus and an image decoding apparatus, is referred to as the image processing apparatus hereafter.